Relieve the pressure in the keg by opening the pressure relief valve, remove the bail, and toss in your hop sack. Now onto the serious stuff.
It is important to purge the headspace with CO2 before pressurizing the keg. Open the pressure-relief valve by pulling on the valve ring and turning the ring 90deg to lock the valve open.
To purge the keg, set the tank regulator to about 30 psi and connect the gas to the IN side of the keg. Let the gas flow into the keg for about 30 seconds or so, then close the relief valve. If your keg doesn't have a relief valve in the lid, an alternative purging technique is to leave the lid unsealed to allow the keg to vent. You will need to reduce the purge pressure to about 5 psi to reduce splashing. Once the headspace has been purged (it should only take a minute or two), seal the lid. You're now ready to carbonate.
This will prevent the potential for oxidation.